1. Field of the Invention
The present invention relates to secondary suspension systems for trucks and, more particularly, to a truck cab suspension system providing a high degree of noise and shock isolation.
2. Description of the Prior Art
The load carried by a truck is supported by a frame which is resiliently carried by suspension springs from the truck""s axles. The primary axle suspension springs usually have very high spring rates which make the ride jarring to the operator and to structures supported by the frame. To dampen the vibration and shock transmitted to the cab, secondary suspension systems have been employed utilizing conventional suspension components such as air springs to isolate the cab structure from shock and shock absorbers to dampen motion of the cab, respectively. Shock and vibration isolation improves occupant comfort and reduces stress fatigue to the cab structure.
Secondary suspension systems connect the cab to, and support the cab from, the vehicle chassis. The secondary suspension system must support the cab""s weight, guide vertical, lateral and fore to aft movement of the cab, improve ride comfort by isolating the cab from vibrations and dampening those vibrations, absorb collision energies, and if desired, provide cab leveling.
Early cab suspension systems placed hard rubber element directly between the cab and the vehicle frame. This sort of minimal suspension system proved uncomfortable for the vehicle""s occupants and could place undesirable force levels on the cab promoting premature structural failure.
Later cab suspensions provided differentiated forward and aft suspension sub-systems making use of a rear suspension sub-system having air springs or coil springs, a guide element based on a panhard rod or other lateral locators, and a dampening element. The air spring and damping elements of the rear suspension sub-system do not provide fore to aft constraints to cab movement. This forces the use of very stiff forward rubber elements which exhibit poor noise isolation.
Europe has favored cab over engine designs due in part to the tight space constraints of many European roads. Typical linehaul European trucks have increasingly been equipped with a cab suspension having suspensive and damping elements located outside of the chassis siderails, that is, toward the outside edges of the truck cab. The suspension element, e.g. the spring, and the damping element, e.g. the shock absorber, are provided as a combined strut, one strut being located at each of the four outer corners of the cab. The top of the strut is attached to the bottom of the cab by a rubber bushing. See Gross, et al., Development of a 4-point-Air Cab Suspension System for Conventional Heavy Trucks (SAE Technical paper 2001-01-2708). However, the weight of the cab is suspended through the primary springs and the bushings, placing the bushings under compression. This has, in practice, required the provision of higher spring rate bushings than would be required if the bushings were not under constant compressive loading.
According to the invention there is provided a vehicle having a suspended cab. The vehicle has a chassis with elongated side rails. The truck cab suspension comprises a plurality of support points for the truck cab positioned on an underside of the truck cab, including two forward support points and two aft support points located near corners of the truck cab. At least two spring-strut units, one each attached to the two aft support points provide for support of the aft portion of the cab. Each spring-strut unit comprises a spring element, a shock dampening element, a yielding bushing interposed between the shock dampening element and the respective aft support point for the spring-strut unit, and a non-yielding brace element interposed between the spring element and the support point. For a four point suspension the truck cab suspension additionally comprises the two forward support points to the truck cab being disposed on the underside of the truck cab substantially forward from the rear support points and toward outside side edges of the truck cab. Two forward spring-damper strut units are attached, one to each of the forward support points. The spring-damper strut units each having a spring element, a shock dampening element, a bushing interposed between the shock dampening element and a respective forward support point, and a non-yielding mount interposed between the spring and the respective forward support point.
Additional effects, features and advantages will be apparent in the written description that follows.